Cut the leaves from the stems of the bok choy. Trim the stems as necessary, then cut them both into roughly 1-in pieces (keeping leaf pile separate from stem pile); rinse everything well.

Heat a large skillet or wok until it looks hazy over the surface, 2-4 minutes. Add peanut oil and swirl it to coat the pan. Add bok choy stems and cook, stirring occasionally, until they just lose their crunch, about 3 minutes.